1. Field of the Invention
The systems and methods of this invention generally relate to communication systems. In particular, the systems and methods of this invention relate to equalization using decision feedback.
2. Description of Related Art
In multicarrier modulation, a transmission channel is partitioned into a multitude of sub-channels, each with its own associated carrier. In implementations of multicarrier modulation known as discrete multitone (DMT) transmission, or orthogonal frequency division multiplexing (OFDM), the generation and modulation of the sub-channels is accomplished digitally, using an orthogonal transformation on each of a sequence of blocks, i.e., frames, of the data stream. A receiver performs the inverse transformation on segments of the sampled waveform to demodulate the data. In the implementation of DMT used as the signaling standard for asymmetric digital subscriber lines (ADSL), the transforms used for demodulation and modulation are the Discrete Fourier Transform (DFT) and its inverse, respectively. Further information regarding the asymmetric digital subscriber line standard can be found in the article Asymmetric Digital Subscriber Line (ADSL) Metallic Interface, ANSI T1E1.4/94-007R8, 1994, incorporated herein by reference in its entirety.
In another implementation, referred to as discrete wavelet multitone (DWMT) transmission, a discrete wavelet transform and its inverse are employed as discussed in M. A. Tzannes et al, xe2x80x9cThe DWMT: A Multicarrier Transceiver for ADSL Using M-Band Wavelets,xe2x80x9d ANSI Standard Committee T1E1.4 contribution 93-067, March 1993, M. A. Tzannes, xe2x80x9cSystem Design Issues for the DWMT Transceiver,xe2x80x9d ANSI Standard Committee T1E1.4 contribution 93-100, April 1993 and M. A. Tzannes et al, xe2x80x9cDMT Systems, DWMT Systems and Digital Filter Banks,xe2x80x9d Proc. ICC 1994, all of which are incorporated herein by reference in their entirety.
Thus, in a multicarrier system, a communication path having a fixed bandwidth is divided into a number of sub-bands having different frequencies. The width of the sub-bands is chosen to be small enough to allow the distortion in each sub-band to be modeled by a single attenuation and phase shift for the band. If the noise level in each band is known, the volume of data sent in each band may be optimized by choosing a symbol set having the maximum number of symbols consistent with the available signal to noise ratio of the channel. By using each sub-band at its maximum capacity, the amount of data that can be transmitted in the communication path is maximized.
In practice, such systems are implemented by banks of digital filters which make use of Fast Fourier Transforms (FFT). In the case in which a single data stream is to be transmitted over the communication path is broken into M sub-bands, during each communication cycle, the portion of the data stream to be transmitted is converted to M QAM symbols chosen to match the capacity of the various channels.
The time domain signal to be sent on the communication path is obtained by selecting a QAM point on each sub-carrier and then adding the modulation carriers to form the signal to be placed in the communication path. This operation is normally carried out by transforming the vector of M symbols via the inverse Fourier transform to generate N, where N represents the size of the transform, time domain values that are sent in sequence on the communication path. At the other end of the communication path, the N time domain values are accumulated and transformed via a Fourier transform to recover the original M symbols after equalization of the transformed data to correct for the attenuation and phase shifts that may have occurred in the channels.
One type of problem encountered in transmission systems is intersymbol interference (ISI). When the time domain values are transmitted, the values are spread over time by the impulse response of the system. Often, a guard band is included to prevent previous frames from interfering with subsequent frames, but these guard bands are often too small to be sufficient on their own. Also, values from within the same frame can interfere with each other to cause ISI, sometimes referred to as intersubchannel interference. The time domain equalizer works to shorten the overall length of the impulse response but usually does not remove all of the ISI.
Therefore, the symbol decoded by the subscriber will include interference from other symbols in other sub-bands and/or earlier or later symbols transmitted in the subscriber""s sub-band. This type of interference is further aggravated by the high side lobes in the sub-bands provided by the Fourier transform. Further information regarding multicarrier transmission systems can be obtained from U.S. Pat. No. 5,636,246 entitled xe2x80x9cMulticarrier Transmission System,xe2x80x9d incorporated herein by reference in its entirety.
For an ideal transmission channel, the receiver transform output is a replica of the modulating data, due to the orthogonality (Nyquist) properties of the particular transform used. However, without compensation, as discussed above, the practical channels can contain severe intersubchannel and interframe interference. That is, the receiver transform output for sub-channel m1 and frame i1 has a contribution not only from si1m1 but also from smi for {m, i}xe2x89xa0{m1,i1}, where sim denotes the symbol transmitted in sub-channel m for frame i. For sake of clarity, in the following disclosure a distinction between intersubchannel and interframe interference will not be made, but rather the combination of the two referred to as intersymbol interference (ISI). However, it is to be appreciated that the receiver transform outputs can also have contributions from independent background noise, which, also for sake of clarity, will be disregarded for this discussion.
Multicarrier systems typically employ equalization to compensate for the effects of ISI. Such equalization is typically done in the time-domain and in the frequency-domain. For time-domain equalization (TDQ), an adaptive filter is trained then applied to the sequence of samples at the receiver, before the sequence is passed to the receiver transform. For frequency-domain equalization (FDQ), processing is employed on the receiver transform outputs.
Let Sm1i1 denote the actual transmitted symbol, and let       s          i      1              m      1        ⋀
denote the FDQ output for subchannel m1 and frame i1. The desired net effect of TDQ and FDQ is for       s          i      1              m      1        ⋀
to be equal to Sm1i1, plus a very small contribution from ISI. The receiver can make a decision about the value for sm1i1 by quantizing       s          i      1              m      1        ⋀
to the nearest constellation point. This decision will be denoted by di1m1. 
Typically, the time-domain equalizer is relied on to perform the bulk of the ISI mitigation, with the frequency domain equalization being used only to perform a phase and amplitude correction for the channel/TDQ combination at the given sub-channel center frequency. In these schemes, each FDQ is implemented as a single-tap complex multiply, applied to the associated sub-channel output.
However, as discussed in U.S. Pat. No. 5,636,246, additional ISI suppression can be obtained by allowing each FDQ to have multiple taps, and combining the receiver transform outputs for several neighboring sub-channel, frame pairs. However, a further reduction in ISI can be achieved by incorporating feedback from one or more neighboring sub-channel, frame pairs in the frequency-domain equalizer combiner.
Accordingly, aspects of the invention relate to reducing intersymbol interference.
Additional aspects of the invention relate to reducing intersymbol interference through the use of feedback.
Additional aspects of the invention relate to reducing intersymbol interference through the use of feedback in a multicarrier environment.
Aspects of the invention further relate to combining multiple FFT outputs as well as decision feedback to create an estimate of a transmitted QAM symbol.
Aspects of the invention additionally relate to using a multi-tap frequency domain equalizer with decision feedback to minimize intersymbol interference in a multicarrier modulation communication system.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of the embodiments.